Traveling wave tube



w FIGJ.

Feb. 16, 1960 F. 1.. WASHBURN, JR 2,925,515

TRAVELING WAVE TUBE Filed June 2a, 1956 2 Sheets-Sheet 1 WWW/l INVENTOR: FREDERICK L. WASHBUiRN,JR.'

HIS A TORN Y.

Feb. 16, 1960 Filed June 28, 1956 F. L. WASHBURN, JR

TRAVELING WAVE TUBE 2 Sheets-Sheet 2 INVENTOR: FREDERICK L. WASHBURN JR.

BYWW

HIS ATT NEY.

United States TRAVELING WAVE TUBE Application June 28, 1956, Serial No. 594,456 16 Claims. (Cl. 315-35) This invention relates to traveling wave tubes and more particularly to improvements in traveling wave tubes adapted for operation at high power levels.

A type of traveling wave tube in common use heretofore, includes a cathode for providing a source of electrons, means for forming the electrons into a beam and means for directing the electron beam axially through a helix, or other type slow-wave structure and input and output coupling means disposed near the ends of the helix. Glass rods or tubing space the helix away from the walls of an outer glass tubing and support it centrally about the path of the electron beam. When the traveling wave tube is used as an amplified, attenuating material in the form of graphite coating may be disposed on the rods or tubing.

Traveling Wave tubes of the above type have heretofore been provided for amplification at relatively low powerlevels. I have found that, before such traveling wave tubes can be utilized for operation at high power levels and at high frequencies, for example, to provide outputs in the order of 100 watts at 3,000 mc., various features of the tube have to be improved in order to render the tubes efiiciently operable under these conditions. For high power levels and high frequency operation, a sharply defined and uniform cross section beam is required. The beam must be accurately aligned with respect to the helix, and means must be provided for dissipating the heat energy developed in the helix. Although the description hereinbelow is directed particularly to a helix, other types of slow-wave transmission line structures could be satisfactorily employed.

It is, accordingly, a principal object of the present invention to provide a traveling wave tube adapted for operation at high power levels.

It is another object of the present invention to provide a traveling wave tube inpluding novel beam forming and controlling means whereby said tube is more efiiciently operable at high power levels.

It is another object of thepresent invention to provide an improved heater for cathodes of traveling wave tubes.

It is another object to provide an improved slow wave structure comprising a helix having improved heat radiating means associated therewith.

In the attainment of the foregoing objects of my invention, I provide, in a traveling wave tube, means for'accurately aligning a helix with the axis of the electron beam. The helix consists of a ribbon-like metallic material and is supported by a quartz tubing. A coating of a carbon suspension is applied to the helix for purposes of radiating heat energy. Further, I provide a toroidal coil heater for the cathode, which heater coil is selfsupporting and wound such as to minimize external magnetic fields developed by said heater to avoid deleterious fi t f such fie ds aths. beam t dvan a e w l c me p r n of he PqifiQati0n a d i g atent G i ggg 22,Z5,5l6 Patented Feb. 16, 1960 Figure 1 is a schematic view of a traveling wave tube embodying features of my invention;

Figure 2 is an enlarged cross sectional view taken along lines 2&2 of Figure 1;

Figure 3 is an .enlarged plan view showing a to'roidal 'heatersaccording to the invention;

Figure 4 is a fragmentary enlarged view partly in cross section of a helix in accordance with my invention;

Figure 5 is a fragmentary enlarged view partly in cross section showing the cathode-aligning method and apparatus in accordance with my invention; and

Figure 6 is an exploded elevation view showing the traveling wave tube and the capsule aligning structure.

In Figure 1 a traveling wave tube 11 is shown disposed in a support or capsule member 25. Traveling wave tube lli-includes a cathode 37 heated by means of heater 55 to provide an electron beam, not shown, which beam is focused and directed by a pole piece 33 and by magnetic means of any suitable conventional design, not shown, through a slow wave transmission line structure 13 consisting of an elongated ribbon type helix to impinge on an electron collector anode 19. Coupling means 15 supported by capsule 2 5 and which may consist of helical windings in concentric relation to helix 13 couples an input electrical signal to the end of the helix 13 nearer the cathode 37. A second coupling means 17, similar to coupling means 15, couples the output signal from the end of the helix,- 13 nearer the anode 19.

The helix 13 provides a high frequency electrical transmission Path through the tube 11 and is designed so that the phase velocity of an electrical wave introduced by coupling means 15 is substantially the same as the velocity of the electron beam projected through the helix 13 so that by virtue of the energy interaction with the electron beam, the electrical Wave is amplified and coupled out by eoupling means 17. Collector anode 19 consists of a tubular metallic member having an open end thereon which is disposed adjacent one end of helix 13 and which is sealed to one end of glass envelope 27. The opposite end of mode 19 is tapered to form a closed end and provides a means to dissipate residual electron energy. The open end of anode 19 is afiixed to an elongated tubular envelope 27 which may be of glass or other suitable material. Anode 19 together with envelope 27, sleeve 47 and bulb 49 form an evacuated container within which are located the cathode 37 and beam controlling elements 56 part of which are atfixed to base supports 69 and 71. Metallic tubular member 63 provides a magnetic shield for the cathode 37 and the beam controlling elements.

Capsule member 25 consists of a partially hollow metallic tubular envelope including member 24 supporting ring member 22 which centrally lo'cate and align anode l9. and envelope 27 within capsule 25. included within capsule 25 are cooling means 18, 'insulative supports and terminations, not shown, for input and output coupling means 15 and 17, respectively. Cooling means 18, of which only the end portions are shown, may be of any design adapted for cooling a traveling wave tube and may, for example, consist of pipes 18' adapted for passing liquid adjacent anode 19, and of pipes 18" which direct air along the length of the helix 13 and the cathode 37 portion of the tube 11.

The cathode end of thetraveling wave tube 11 is shown in more detail in Figure. 2. The cathode 37 having an electron-emissive coating on a surface 38 thereof is heated by means of toroidal heater 55 which is. wound in a manner to eifect more eflicient operation, to be described in detail hereinbelow. Accelerating anode 39 characterized by a helical or Brillouin type of flow. To

obtain proper helical flow, the electrons produced by the cathode 37 must, prior to entering the helix 13 region, be formed into a beam which enters the helix region at a predetermined point 'by the accelerating anode 39 and the focusing electrode 43. The electron beam entering the helix 13 passes through an aperture 40 in magnetic pole piece 33 which further effects beam control.

The pole piece 33 with its frusto-conical protrusion 34 is useful to provide the proper field conditions, as the beam enters the helix 13 region to obtain the aforesaid helical flow. The beam of electrons is directed through the center of helix 13 which, as above noted, is disposed along the length of the tube 11 and supported within envelope 27 by quartz, or the like, tubular support 29. Attenuating material, not shown, necessary for proper operation of the traveling wave tube as an armplifier may be disposed on the exterior wall of the quartz tubing 29. Helix 13 is aligned with aperture 40 by inserting the helix and the enclosing support 29 into snug fit with an open end section of a helix base support 31 which is centrally afiixed to a reference surface on the same side of magnetic pole piece 33 as the frusto conical protrusion 34 as by spot welding.

Initially, and as will be discussed in detail subsequently, the helix 13 is centered and aligned at the end adjacent the cathode 37 and then the helix 13 and the helix containing tubular envelopes 27 and 29 are inserted into capsule and the anode '19 end of the helix 13 is constrained and aligned by ring member 22 and support member 24. Since the tube 11 is intended for high power operation, very accurate alignment of the axis of the helix 13 with the axis of the electron beam is necessary, otherwise electrons impinging on the helix may cause undesired hot spots on the helix, thus undesirably affecting proper operation of the tube.

The assembly of the cathode end of the tube, in order to provide coincidence of the axes of the cathode 37, accelerator 39, pole piece 33, and helix 13 is next to be described. As may best be seen in Figure 5 a cylindrical metallic sleeve 47 having an accurately machined ring-shaped shoulder member 48 affixed to the inner wall thereof, is itself vacuum sealed to the glass bulb 49. Shoulder member 48 serves as a reference surface for positioning the cathode 37 and the pole piece 33.

to be in coincidence with pole piece 33 and, in fact,

does form part of the pole piece 33 magnetic structure. The surfaces of member 36 serve as reference surfaces for aifixing and aligning the tube 11 within capsule 25 as will be detailed hereinbelow.

The cathode 37 subassembly which, as shown in Figures 1 and 2, includes focusing electrode '43, heater 55 and support members 54 and 62 is initially mounted as a unit onto glassbase 49' through supports 69 and 71 which are affixed to pin members 73 and 75 which are in turn vacuum sealed to glass base 49. To position the cathode 37 subassembly within bulb 49 a cylindrical jig 65 (Figure 5) conformable to sleeve 47 and having a center rod extension 67, is inserted in sleeve 47 to bear against shoulder 48 in an accurately formed fit.

The cathode 37 subassembly is inserted in bulb 49 and centrally disposed aperture 45 in cathode 37 receives the rod 67 thereby aligning the cathode 37. A stop 68 carried by rod 67 locates the focusing electrode 43 and the cathode 37 a predetermined desired distance from permit this alignment.

4 shoulder 48. The base 49' is then sealed onto bulb 49 and the cathode 37 subassembly is thus located in position. Jig 65 is then withdrawn.

Referring again to Figure 2, the pole piece 33 having accelerator electrode 39 and support 41 atfixed thereto is next fitted into the metallic sleeve 47 and affixed to shoulder member 48 as'by screws 50. The pole piece 33 also has affixed thereto a helix base member 31 which is of cylindrical construction having a reduced or step section 30 against which helix support 29 butts. Base member 31 is aflixed in coaxial relation with the truncated cone 34; helix 13 having previously been attached as by spot welding to said base member 31. Quartz tubing 29 is then slipped over helix 13 and into a snug fit with base member 31.

The outer glass tubing 27 is affixed to cup shaped flange member 35 which member is formed to provide accurate mating surfaces with relation to the pole piece 33 and wtih sleeve 47. Glass tube 27 is next slipped over quartz tubing 29 and tubing 29 is supported by a shoulder 26 on anode 19 so as to be in predetermined spaced relation from tube 29 along the entire length thereof, Figure 1.

Referring to Figure 6, the collector anode 19 end of the helix 13 is positioned to have the beam axis and the helix axis coincident by inserting the tube 11 within capsule 25 and affixing, as by screws 52 (Figure 2), member 36 in recessed portions 28 and 40 which are formed to receive member 36 in conforming relation therewith. The anode 19 is inserted through ring member 22 in support 24 and constrained into alignment with the cathode end of the helix and the beam axis cup shaped flange member 35 being sufliciently flexible to The two opposite ends of helix 13 are thus accurately aligned with the axis of the electron beam provided by cathode 37.

To obtain optimum operation of a traveling wave tube, the electron beam should be uniform in cross section. For this purpose, I provide the cathode 37 having a concave surface 38 which tends to direct the electrons toward a common center point. The inherent mutual repulsion characteristics of the electron beam is thus partially compensated for, and a more uniform cross section beam results. Further, cathode 37 is provided with a recess 51 which is formed such that the outer portions of cathode 37, and in particular the outer edges thereof, obtain from heater 55 a larger supply of radiated heat. Due to the larger mass of metal in these outer portions good heat conduction is obtained so that no portion of the cathode can be appreciably hotter than any other portion. Since the outer portions of cathode 37 are also subject to more heat losses due to radiation of heat to adjacent elements and due also to the fact that the outer portions have larger heat radiating surfaces, the additional supply of heat is required to maintain a uniform temperature throughout emitting surface 38 so that an electron beam of uniform cross section can be provided.

-I have found that the cathode heaters heretofore used, develop magnetic fields in the beam forming region which adversely afiect the electron beam. To correct for the above, I provide a novel cathode heater which provides the necessary heat to the cathode but does not adversely affect the electron beam. 1

To provide a proper heating effect throughout the cathode 37, the heaters should be spaced from other elements and should preferably be self-supporting since any'materials used to support the heaters will cause an increase in the amount of desirable gases introduced into the tube.

In Figure 3, there is shown an air core toroidally wound coil 55 in accordance with my invention. A first lead 53, supported by insulative member 54, Figure 2, is connected to coil 55. From the point where the lead 53 is connected to the coil, a first portion 57 of the coitisswound ima leit handisense or clockwisejirection, andasecond portion 59,.of .the 'coil,is. ..wound. in a ,right hand... sense or counter clockwisefdirectionc A second lead .61, alsosupported by.memberQSA FigureZ is, con: nected. to. the coil at a. diametrally opposite... point. from the=.first..-lead 53. Thetwo portionsw57 and59 of the coil 55 are..thus..wound...to have..opppsing.sense and. the magnetomotive forces produced. by, thetwo. portions. .of thewindingsare, in. a. direction. .as..shown.by. the arrows, in.aiding or.complementary relation to. each. other... The magneticfield developed. by thecoil.:55 is substantially confined to the region. within the toroidal .winding. and onlysnegligible external fields, are .developed. Good'suppprt is. provided by two leads.53. and..61..which are suffiv cientlyrigidto maintainthe toroid.55 apredetermined distance. from thecathode .372

In... Figure 4, a. portion of. the. ribbon .helix 13. is shown. The helix..13.is made of aaribbontypematerial order to provide. the greatest. amount of .heatdissipating surface and for the purpose ofpresenting theleast amount of surface area on which the axially moving electron .beam can. impinge. Electrons impinging on the helix 13 cause the helix to become heated, which heat may. causeimproper operation. of the traveling wave tube, Since the tube is intended. for high power..opera-. tion, a. large number of electrons flow through the helix. The number of electrons whichimpinge. on the helix 13 shouldbe 'kept ata minimumto prevent the helix 13 from becoming excessively hot... Although the radiating surface is. maximized and the area. susceptible to being impinged by the electronbeamis minimized, due to the large powers involved, the helix still may tend .to run hot and even to develop hotspots. A

" In. order to adapt the helix. 13 for. high. power operation, I.coat the outer surface. 30 ofthe helix 13 with. at material havinggoodheat radiation character. istics, ,such .as. finely divided carbon. A carbon suspension may be. conveniently applied. to.the..helix 13 as by spraying to provide a. coating of-uniformthickness. In one practical. embodiment, wadding ofany suitablernateriaLQnotshown, is firstinserted between the turns. of the.helix.13 to mask the surfaces. on which acarbon coating is not necessary and only. the. outer surface 34 is coated. Carbonapplied to the innersurfaces32does not appreciably contribute to the radiation of heat away fromthehelix 13. Carbon may also be applied .between th'eturns on the sides of the helix, although I.found the application of a uniform coating ofjcarb'on betweenthe turns to be less. convenient than applying the carbon. to only surface 30;

The carbon .coating;0n the:helix--;.13. functions as an excellent heat..ra.diator to. prevent. thehelix from developing hot-spots and will cause the helix to operate at. a low. temperature to provide; good .1 operating conditions for.rthe;traveling..wave .tube- 1 i While. specific. examples havebeenis given in describing details .ofrthis invention,.it willbe understood'that they have beenpgivenmerely by. way. of illustration and that the invention .is not limited. thereto.

What: I. claimasnew and desireto secure by. Letters Patent of the. United..States .isz. J

1. A? traveling wave. tube. comprising-beam. providing meansandrbeam controlling means including a magnetic pole piece'ancLaslow Wave.transmission lineafiixedyat one end thereofto. said. pole. piece; a: collector'electrode afiixed adjacent the. opposite end: of." said transmission line in such a..manner as to providesome relative'movement betweenthe. axis of. said-coll'ector electrode. and the axis of said beam providing means, an encapsuling member for said tube, support means in said encapsuling adapted member to receive said collector, a surface on said encapsuiing member accurately disposed with respect to said support means and adapted to abut a portion of said pole piece, said surface and said collector support means constraining and maintaining said beam controllingmeans.in alignmentmvith saidbeam. providing means; i r

2. A,traveling ,-wave tube, comprising a cathode, acollecton. nodeaand .an elongated slow,. wave transmission lines disposed,along. the. axis: er. an. electron beam. there.- between, means supporting said transmission line, a. magnetic .pole? piece ihaving am-centrally, formed aperture therein, one. end ,of .said Qtransmission. line. affixed. in concentricrelation tto saidlv polerpieceqaperture, the opposite end of said. .transmissionfline.fixed. with relation to said anode, said anode being fixed foltrlimited movement with its..axis..in substantial alignmentwiththe axis of said beam, said magnetic polepiece having bearing surfaces formedthereon, aicapsulemembier,support means in said capsule .having .a. bearing.,P.or.tionadapted,to receive said anode, and recessed portions,in..said..capsule accurately disposed with relation-to. said .bearingportion and adapted to. receive said. pole piece in. conforming relation, said anode support means and said recessed portionseffecting alignment of the axis. of saidtransmission line and the axis of. said .beam.

3. A traveling Wave .tubecomprising a cathode, a collector, anode, a.helix: disposed along, the path of an electron beam therebetween, a first tubular member snugly encasing said, re1ix, ,.a..second-tubular member encasing said first tubular member, a. magnetic pole piece having an. aperture therein, one-eI1d-..of .said,.helix being affixed in concentric relation to said apertureinsaid pole piece, the. opposite. end. of said. -,h.elix being, held substantially concentricwith said, aper,ture .by said, second tubular member, said pole piece, having, reference surfaces formedthereon, said ,secondtubulanmember having one end .fiexibly. affixedto... said. magnetic pole. piece with .its axis, in. substantial. alignment withv the aperture in. said pole 'piece, a, capsule ,m ember adapted to contain said traveling wave tubein, an axi al r ngin said capsule adapted toreceive said,second,.tub11lar member and said anode, and recessed; p ortions ,s d, capsule, adapted. to receive saidreferenlce surfaces, njconfbrming relation, said ring and said recessedfioriions effecting alignment ofsecond. tubular member and: the-said encased helix with thepath ofsaid beam..

4. In theassembly ofatrayelingwave tube including a cathode and a.helix, the method of aligning the helix with relation to the axis of an, electronbearn, comprising positioning areference,surface with respect to the axis of saidlzelix, said reference surface having an aperture in the center thereof, centering thecathode of said tube with the axisof the aperture, positioning the cathode a predetermined distance, frjorr said reference surface,

afiixing the center of the b"ase of 'said, helix coincident with the aperture in said reference surface, encasing. said helix ina first tubular member, encasing said first tubular member in a seco-nd tubular member, inserting said second tubular member in a support capsule having a constraining support adaptedjt receive said second tubular member, and affixing said support capsule member to said reference surface 5. A traveling wave tube adapted for use. at high power comprising a cathode, a collector anode and a ribbon type helix disposedalongthe path .ofan electron beam therebetween, said.helix havin g only the outer surface thereof coated with. a material .of high heat emissivity, means aligning said helix withthe axis of said electron beam, a wire heater for said cathode, saidhcater being self-supporting, helically woundand formed in the shape of a torus, said heater having portions" thereof 'wound in mutually opposing senses; whereby external fields produced by said heater are minimized.

6. A self-supporting wire heater for a cathode, said heater being helically wound and formed in the shape of a torus, respective portions of said heater being wound in opposing senses.

7. A self-supporting helically wound wire heater Wound on an air core and formed in the shape of a torus,

one helically wound portion of said heater wires wound in a left hand sense and a second portion of said heater wires helically wound in a right hand sense such that magnetomotive forces developed by said heater are oriented in the same relation whereby negligible fields external to said air core are produced.

8. A helically wound wire heater shaped in the form 'of a torus, said heater having alternate portions of said wires wound in a right hand sense and other alternate portions wound in a left hand sense to prevent formation of external fields thereby.

9. A slow wave structure for a traveling wave tube comprising a ribbon helix having only the outer surface thereof coated with a material of high heat emissivity for dissipating heat energy from said helix.

10. A slow wave structure for a traveling wave tube comprising a ribbon helix having the outer surfaces only coated with a carbon suspension for radiating heat energy from said helix.

11. A traveling wave tube including a ribbon helix, a thin tubular dielectric support for said ribbon helix, said helix having only its outer surface coated with a material of high heat emissivity for dissipating heat energy from said helix.

12. A traveling wave tube comprising a cathode, a collector anode and an elongated slow wave transmission line disposed along the axis of an electron beam therebetween, said transmission line having a coating of a material of high heat emissivity, means supporting said transmission line, a magnetic pole piece having 'a centrally formed aperturetherein, one' end of said transmission line afiixed in concentric relation to said pole piece aperture, theopposite end of said transmission line adjacent to said anode, an envelope surrounding said slowwave transmission line and aflixed at one end to said anode, said envelope having its opposite end flexibly attached to said pole piece, said magnetic pole piece having bearing surfaces formed thereon, a capsule member, support means in said capsule adapted to receive said anode, and recessed portions in said capsule adapted to receive said pole piece in conforming relation, said anode support means and saidrecessed portions effecting alignment of the axis of said transmission line and the axis of said beam.

13. A traveling wave tube comprising a cathode, a

collectoranode, a helix disposed along the path of an electron beam therebetween, said helix having the outer surface thereof coated with a material of high heat emissivity, a first tubular member snugly encasing said helix, a second tubular member encasing said first tubular member, a magnetic pole piece having an aperture'therein, said second tubular member having one end flexibly fixed to said magnetic pole piece in concentric relation to said aperture, one end of said helix being affixed in concentric relation to said aperture in said pole piece, the

opposite end of said helix being held concentric with said aperture by said second tubular member, said pole piece having reference surfaces formed thereon, a capsule member adapted to contain said traveling wave tube in an axial ring in said capsule adapted to receive said second tubular member and said anode, and recessed portions in said capsule adapted to receive said reference surfaces in conforming relation, said ring and said recessed portions eifecting alignment of second tubular member and' the said encased helix with the path of said beam.

14. A traveling wave tube having an electron gun and a slow wave structure, an envelope enclosing said slow wave structure and said electron gun, said envelope havtilted relative to the other, a reference surface on one sideof said flexible portion, means foraligning the axis of the electron stream provided by said electron gun'with the axis of said slow Wave structure, said meansincluding an aligning member connected to said reference surface and having a support means adapted to constrain the'end of said tube on the opposite side of said flexible conneca tion into alignment with the axis of said electron stream,-

15. In a traveling wave tube including an electron stream producing gun and an elongated slow wavetranse mission line enclosed in an evacuated envelope agrefer ence member external to said envelope and between said electron stream producing means and said slowwave structure, said evacuated enclosure having a fiexibleseetion between said electron stream producing means andsaid slow .wave structure, means for aligning the axis of said slow wave transmission line with the axis'of the electron beam provided by said electron beam producing means, said means comprising a capsule member positioned around said slow wave transmission 'line and fixed to said reference surface, means including a support means adapted to receive the end of said enclosure op posite said electron gun and constrain said enclosure and said slow wave transmission line into alignment with the axis of the electron beam.

16. In a traveling wave tube including an electron gun for producing a stream of electrons and an elongated slow wave transmission line, a magnetic pole piece having'ia centrally located aperture and a reference surface there on, bulb means'enclosing said electron gun and. afiixed to the reference surface of said magnetic pole piece to align the electron stream and the centrallylocated ap; crime in saidmagnetic pole piece, an ,envelopemember' flexibly attached to the. opposite side 'of said pole piece from said electron gun and enclosing said slow Wave struc': ture, said slow wave structure being at least partially sa ported inalignment with the centrally located aperture in said pole piece by said envelope member, means for aligning the axis of said slow wave transmission line and the electron stream provided by said electron gun, said means comprising a capsule member having one end mechanically coupled to said magnetic pole piece, in such a manner as to surround said envelope member, said capsule member including a support means positioned in ternally thereof to support said envelope and constrain said envelope and slow wave transmission line into align; ment with the axis of the electron stream.

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